Sometimes credit cards will allow you to carry two balances -- one for the purchases on which you have the deferred-interest arrangement and one for purchases you make later.

What you might not know: Credit card issuers are required, unless you state otherwise, to put anything above the minimum toward the balance without the deferred-interest arrangement, says Wu. The only exception: During the last two months of the deferred-interest period, card issuers have to direct anything above your minimum to the deferred-interest balance, she says.

You only have a set amount of time to pay off your deferred-interest balance. If your payments are going toward the other balance, you're not making any headway.

One solution: If you're carrying two balances on the card, specify in writing how much goes to each when you send in your bill. The law "allows consumers to allocate" their payments, Wu says.

If you do that, follow up regularly, and make sure the money is going exactly where you want it.

Another solution: Don't run two balances when making use of zero percent financing. Until you pay off the deferred-interest balance, don't add to the confusion (and debt) by making more charges. Use a different payment method for new purchases.